1. Field of the Invention
The present invention relates to a surface emitting semiconductor laser used as a light source for optical communication, an optical recording apparatus, a laser printer or the like, a method of producing the same and a surface emitting semiconductor laser array.
2. Related Prior Art
As a relatively simple and easy method of producing a surface emitting laser, there has been proposed a method in which a surface emitting laser structure layer is stacked on a flat GaAs substrate and thereafter ions (protons) are injected into a region, where a current is not made to flow, to make the region electrically insulated, a active region is defined and a current constriction structure is formed (Japanese Published Unexamined Patent Application No. Hei 4-226093).
This method has a fault that, while a surface of an element is flat and an electrode and the like are easy to be formed, an optical wave guiding (index guiding) structure which is important in a laser structure is not formed. This laser is called a gain guiding surface emitting laser but laser characteristics are low, for example, a threshold current is high, a quantum efficiency is low and the like. Besides, it has a problem of a thermal lens effect that a micro fluctuation of a refractive index arises in the bulk of a laser under influence of heat generation and such a micro fluctuation causes gradual increase in an optical output, so that the output is hard to be constant. Moreover, it has another problem that it is required that a bias current is always made to flow in order to realize a high speed modulation, so that power consumption is large.
As a method of forming a current constriction structure without usage of proton implantation or the like, there has been proposed a method in which a surface emitting laser structure layer is formed in a layered manner on a surface of a substrate processed in a concave shape (Japanese Published Unexamined Patent Application No. Hei 5-226778).
This method is to utilize a phenomenon that a silicon doped layer grown on a (111) oriented surface of a substrate which is processed in a concave profile whose bottom is in the shape of a flat regular triangle comprises n-type and p-type layers respectively grown on the two kinds of surface, that is flat and sloped surfaces. That is to say, after a n-type multilayer mirror is grown, a n-type active layer is grown. Since an active layer on the flat surface is of n-type and that on the sloped surface is of p-type, an active layer is surrounded by a pn junction in a lateral direction. A surface emitting laser structure is completed by growing a p-type multilayer mirror thereon.
Even in this method, in the same way as that of the above mentioned example, an index guiding structure is not formed but a gain guiding surface emitting laser is formed, so that there is a problem that laser characteristics are low.
In this structure, since a region of a pn junction which has the minimum band gap is a corner region between the flat and sloped surfaces of the active layer, an electron and a hole are subjected to recombination to emit light. In the case of a surface emitting laser, since recombination desirably occurs in the middle portion of the flat surface, there arises a problem that a laser of this structure has low characteristics.
There has been proposed an index guiding laser to solve a problem of a gain guiding laser (Electronics Letters, Vol. 31, No. 11, 1995, pp. 886 to 888). This laser has a current constriction structure and a light wave guiding structure formed by selectively oxidizing only an AlAs layer inserted near an active layer to make it electrically isolated. After growth of a laser structure on a GaAs substrate 1, a circular cylinder of 100 .mu.m to 10 .mu.m in diameter or an angular prism of a similar size is formed in a protruding manner by etching to expose a sectional surface of an AlAs layer. Thus etched substrate is heated in a water vapor atmosphere to oxidize the AlAs layer on the outer side surface thereof. An oxidation of the AlAs layer advances from the outer side surface of the circular cylinder or angular prism to the center thereof while an oxide is formed in a doughnut shape. An oxidizing AlAs forms aluminum oxide 18. This has a high insulating property and thereby blocks a current, so that a current flows only through an aperture of an unoxidized region in a doughnut-shaped aluminum oxide. Since aluminum oxide has a low refractive index and thus there is formed a light wave guide along a light emitting direction (a longitudinal direction), the light wave guide being a region of the aperture of the doughnut working as a core. This structure is shown in FIG. 7. In this laser element, a current is made to flow between an n-type electrode 13 formed on a rear side of a n-type substrate and a p-type electrode 12 formed on a top end surface of the laser.
An index guiding surface emitting laser using such selective oxidation of AlAs has excellent laser characteristics such as a low threshold current, high quantum efficiency, good high speed modulation property and the like. However, since the circular cylinder or angular prism is as high as 3 .mu.m or more in a protruding manner, formation of an electrode on the top end surface is difficult. Moreover, since the top end surface is generally tens of .mu.m square (a circle of tens of .mu.m in diameter) or smaller in area and has a light emitting apparatus opened in the central portion, wire bonding cannot be applied to a metal electrode on the top end surface. For that reason, an interconnect to the electrode is extended over a surface of the substrate and an electrode pad is formed for wire bonding at the other end of the interconnect located in a part other than a part which a laser element occupies. Such a high protrusion is a great disturbance to form an electrode interconnection in its process. There has been a great hardship in film formation for an electrode, coating of resist, exposure to light, electrode etching and the like over a step having a rise of 3 .mu.m or more in a concavity and convexity profile with a good yield.
As described above, a conventional surface emitting laser has suffered from serious problems.
Besides, in the case of a matrix interconnection surface emitting laser array, such problems are more serious.
The matrix interconnection laser array has a structure wherein surface lasers are two dimensionally arrayed and individual arrays are driven by positive electrodes and negative electrodes interconnected in row and column directions. When a degree a integration of a laser is higher, since individual interconnection of electrodes on lasers cannot be realized, matrix interconnection becomes necessary. In matrix interconnection, grooves of several .mu.m or more in depth have to be formed by etching in gaps between adjacent columns of lasers in order that they are electrically isolated between any adjacent two thereof regardless of a gain guiding structure or a index guiding structure. Negative electrodes are formed in parallel to the grooves and positive electrodes are formed on the negative electrodes in directions perpendicular to the array columns with an insulating film interposing between them. It has been found from our experiments that formation of matrix interconnection on such a deep concavity and convexity profile is a very difficult task.
Even in the most advanced LSI process, a depth of a concavity and convexity profile is generally on the order of 1 .mu.m. There has been no proposal of a technique in which interconnection of electrodes are formed across a surface having a concavity and convexity profile and as can be seen from such circumstances, such a electrode interconnection technique is extremely difficult. As a matrix interconnection surface emitting laser array, an example in which a surface emitting laser is fabricated by matrix interconnection with ion implantation has been proposed (AT & T Bell Laboratories, IEEE Photonics Technology Letters, Vol., No. 8, August 1994, pp. 913 to 917). Ion implantation is applied to an upper side mirror to form a current constriction structure, then the laser structure is etched off to a depth of several .mu.m up to the vicinity of the lowest layer thereof and a residual layer is further electrically isolated by ion implantation. Thereafter, as shown in FIG. 8, positive and negative electrodes are formed by matrix interconnection.
In this case, while a gap between each adjacent column of a laser array is insulated by ion implantation, since a depth, to which insulation can be achieved by ion implantation, is on the order of 4 .mu.m, a depth of several .mu.m has to be removed by etching, so that there is still a concavity and convexity profile of the order in the range of 3 to 5 .mu.m on the surface. It is extremely difficult to form negative and positive electrodes by matrix interconnection over a surface having a depth in a concavity and convexity profile. That is to say, since an accuracy in patterning by photolithography cannot sufficiently be achieved in an actual fabrication process, there arises with ease a short between electrodes, breaking of interconnection, remaining of resist and the like, which makes it very difficult to fabricate a good laser array. While, according to the teachings of the article, an array pitch of the laser array is 140 .mu.m in both row and column directions, if the pitch is narrowed to tens of .mu.m the matrix interconnection is more difficult with an increased extremity, which makes it possible to form the matrix interconnection by this method.
Especially, when an index guiding surface emitting laser is integrated, a protruding profile such as a circular cylinder and angular prism is formed on a laser in order to conduct oxidation of AlAs and the like and then etching has to be applied to remove up to the lowest layer of a laser array in order to achieve electrical insulation of the laser array. It is extremely difficult to form a matrix interconnection on a surface of a concavity and convexity profile in such a deep and complicated manner like this. When an array pitch of the laser array is narrowed to tens of .mu.m, difficulty is further increased.
As mentioned above, there have been problems that a conventional surface emitting laser having a flat front surface is a gain guiding laser without a light wave guide and its laser characteristics are low.
On the other hand, an index guiding surface emitting laser has a deep concavity and convexity profile on the surface of an element and further hardship in formation of electrode interconnection and the like.
In addition, a conventional matrix interconnection surface emitting laser array has also a problem that since a deep concavity and convexity profile is produced by an etched groove which works as insulation between laser arrays, matrix interconnection over the profile is extremely difficult.